The present invention relates to a voice coil motor to be used, for example, as means for positioning a magnetic head in a magnetic disk apparatus, and so on, and more particularly to an improved voice coil motor whose components can be fabricated at a low cost with a high accuracy.
Voice coil motors have conventionally been used in various applications because of their simple structures and their excellent performances as driving means for high-speed straight or swinging movements. In a magnetic disk apparatus, in particular, which is required to effect quick and accurate movements of a magnetic head from one track position to another, voice coil motors, which need less access time than other types of motors, have widely been used as means for positioning the magnetic head (see for instance, U.S. Pat. No. 4,505,055, Japanese Patent Laid-Open Nos. 64-89946 and 4-311888).
FIG. 1 is a plan view illustrating a voice coil motor to which the present invention is applied. In FIG. 1, reference numeral 1 denotes an E-shaped yoke made of a soft-magnetic material such as soft iron, which comprises an arcuate center yoke portion 2 and arcuate side yoke portions 3, 3. Secured to the open end of the yoke 1 with screws, etc. is a counter yoke member 4 constituted by a flat plate made of a ferromagnetic material.
Numeral 5 denotes permanent magnets each having an arcuate cross section and magnetized in its radial direction. The permanent magnets 5, 5 are secured to inner sides of the side yoke portions 3, 3 by an adhesive, etc. with the same magnetic pole adjacent thereto, and magnetic gaps 6, 6 are provided between the center yoke portion 2 and the permanent magnets 5, 5. Numeral 7 designates an arm having one end to which a hollow rectangular-cylindrical movable coil 8 is secured and having the other end to which a function member such as a magnetic head (not shown) is secured. The arm 7 is swingably or pivotally supported by a shaft 9 such that the movable coil 8 is located in the magnetic gaps 6 between the permanent magnets 5 and the center yoke portion 2. Numeral 10 refers to a plurality of screw holes opening at one surface of the yoke 1.
When a current is supplied to the movable coil 8, a driving force around the shaft 9 is produced by the movable coil 8 according to the Fleming's left hand rule. This force causes the arm 7 to pivot or swing to bring the magnetic head at the other end of the arm 7 to a desired recording track on a magnetic disk. The direction of the pivotal or swinging movements of the arm 7 can be changed by inverting the direction of a current applied to the movable coil 8.
The yoke 1 for the above-described voice coil motor is conventionally fabricated from an iron-based, ferromagnetic material as described above by a forming method such as cold forging, powder metallurgy, and precision casting. The concave and convex surfaces of the center yoke portion 2 and the side yoke portions 3 constituting the yoke 1 should be mechanically worked to a dimensional tolerance of 0.05 mm or less, for example, in order to ensure a high mounting accuracy of the permanent magnets 5 and a high dimensional accuracy of the magnetic gaps 6.
However, mechanical working of the concave and convex surfaces of the yoke 1 with a high accuracy is generally much complicated and hence requires a number of working steps and much working time. This invites an increase in mechanical working cost and thence an increase in manufacturing cost of the voice coil motor.
In addition, if the yoke 1 is formed by a method such as precision casting, it would have magnetic properties corresponding to those of S10C, particularly a saturation magnetic flux density Bs of 1.6 to 1.7(T). On the other hand, a steel plate (SS41, SPC, etc.) used to make a flat-type voice coil motor has a saturation magnetic flux density Bs of about 1.8(T), meaning that the mechanical working leads to the degradation of magnetic properties.
Since the yoke 1 has a complicated configuration as a whole as shown in FIG. 1, shaping of a material therefor is difficult and decreases the productivity. Further, since any of the above forming methods needs an expensive die having a complicated shape, the manufacturing cost of the voice coil motor is necessarily high.
Recently, there are more strict requirements for miniaturization and cost reduction of this sort of voice coil motors. The conventional structures, however, increase the manufacturing cost, failing to meet the requirements.